1. Field of Invention
The present invention relates to an image display system. Particularly, the present invention relates to a display system with spatial light modulator(s) including micromirrors controlled to operate with an intermediate oscillating state.
2. Prior Art
As disclosed by the U.S. Pat. No. 5,287,096 the technique by applying the pulse width modulation (PWM) control according to the digital picture data for controlling the micromirrors of a digital mirror device (DMD) for displaying a projected picture is well known in the art. The optical modulation is carried out depending on the digital picture data by balancing the incoming light from a light source to each micromirror between two states. These two states are the ON state when the incoming light is reflected toward a projective optical system and an OFF state when the incoming light deviates from the projective optical system. The luminance of each pixel of a projected picture depends on the total length of time in which each micromirror stays in the ON state in each frame period of the picture. Therefore, there is a technological challenge to process increase amount of digital picture data in one frame period, and a higher mirror speed for modulating and controlling the mirror into the ON state in order to represent images with higher number of gray scales.
Therefore, to present images with higher number of gray scales without increasing the speed of modulation-controlling a micromirror into the ON state, it is necessary to increase or decrease the quantity of light of a light source in addition to the modulation-control by balancing the micromirror in the DMD as disclosed U.S. Pat. No. 5,589,852, thereby complicating the controlling operation.
For these reasons, even though there are significant advances made in recent years on the technologies of implementing electromechanical micromirror devices as spatial light modulator, there are still limitations and difficulties when the image display system implements the electromechanical micromirrors as spatial light modulator to provide high quality images display. Specifically, when the micromirrors are implemented as the spatial light modulator for a color sequential display system to project the display images, the images have an undesirable “rainbow” effect.
Particularly, the rainbow effects become even more pronounced in the display system based on the HDTV format. The HDTV display format becomes popular while the image size for display on a screen becomes ever bigger such as exceeding 100″ in diagonal size. The pixel size on the screen is more than 1 mm when specification is that 100″-size image includes 1920×1080 pixels. Similarly for image displayed on a screen of 50″diagonal-size according to the XGA format, the pixel size is also 1 mm. For such larger size of display pixels, an observer can see each of the pixels on the screen. For these reasons, the display systems require a high number of gray scales of more than 10 bit or 16 bit in order to eliminate the rainbow effect to provide a high quality display system. Furthermore, when the display images are digitally controlled, the image qualities are adversely affected due to the fact that the image is not displayed with sufficient number of gray scales.
Electromechanical micromirror devices have drawn considerable interest because of their application as spatial light modulators (SLMs) that can be conveniently digitally controlled. A spatial light modulator requires an array of a relatively large number of micromirror devices. In general, the number of devices required ranges from 60,000 to several millions for each SLM. Referring to FIG. 1A for a digital video system 1 disclosed in a reference U.S. Pat. No. 5,214,420 that includes a display screen 2. A light source 10 is used to generate light energy as an illumination light source for displaying an image on a display screen 2. The light 9 projected from the light source is further focused and directed toward a lens 12 by a mirror 11. Lenses 12, 13 and 14 function as a beam columnator to columnate the light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer through data transmitted over a data cable 18 to selectively redirect a portion of the light from a path 7 toward a lens 5 to displaying on a screen 2. The SLM 15 has a surface 16 that includes an array of switchable reflective elements, e.g., micromirror devices 32, such as elements 17, 27, 37, and 47 as reflective elements attached to a hinge 30 shown in FIG. 1B. When element 17 is in one position, a portion of the light from the path 7 is redirected along a path 6 to lens 5 where it is enlarged or spread along a path 4 to impinge on the display screen 2 so as to form an illuminated pixel 3. When element 17 is in another position, the light directed away from the display screen 2 and hence pixel 3 is dark.
The on- and off-states of micromirror control scheme as implemented in the U.S. Pat. No. 5,214,420 and by most of the conventional display system impose a limitation on the quality of the display. Specifically, an application a conventional configuration of a control circuit is faced with a limitation that the gray scale of conventional system with the micromirrors controlled by applying a pulse-width modulation (PWM) between an ON and OFF states, is limited by the minimum controllable amount of incremental illumination determined by the LSB (least significant bit, or the least pulse width). Due to the On-Off states implemented in the conventional systems, there is no way to provide shorter pulse width than LSB. The least amount of incremental brightness controllable by the spatial light modulator determines the resolution of the gray scale and that in turn is determined by the light reflected during the length of time controlled by the least pulse width. The limited gray scales lead to degradations of image display.
Specifically, FIG. 1C shows an exemplary circuit diagram of a prior art control circuit for a micromirror according to a U.S. Pat. No. 5,285,407. The control circuit includes memory cell 32. Various transistors are referred to as “M*” where “*” denotes a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5 and M7 are p-channel transistors; and transistors, M6, M8, and M9 are n-channel transistors. The capacitances C1 and C2 represent the capacitive loads presented to memory cell 32. Memory cell 32 includes an access switch transistor M9 and a latch 32a, which is the basis of a static random access switch memory (SRAM) design. Each of the access transistors M9 in a row receives a DATA signal from a different bit-line 31a. Turning on a row select transistor M9 by using the ROW signal applied to a wordline enables an operation for writing data to the memory cell 32. Latch 32a is formed from two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states. State 1 is Node A high and Node B low and state 2 is Node A low and Node B high. The dual state switching operations are carried out by the control circuit to control the micromirrors to move to a position either at an ON or OFF angular orientation as shown in FIG. 1A. The brightness, i.e., the gray scales of display for a digitally controlled image system is determined by the length of time the micromirror stays at an ON position. The length of time a micromirror is controlled at an ON position is in turn controlled by a multiple-bit word. For simplicity of illustration, FIG. 1D shows the “binary time intervals” when controlled by a four-bit word. As shown in FIG. 1D, the time durations have relative values of 1, 2, 4 and 8 that in turn define the relative brightness for each of the four bits where the “1” is for the least significant bit and the “8” is for the most significant bit. In accordance with the control mechanism as shown, the minimum controllable difference between gray scales for showing different brightness is a brightness represented by a “least significant bit” that maintains the micromirror at an ON position.
When adjacent image pixels are displayed with a great degree of different gray scales due to a very coarse scale of controllable gray scale, artifacts are shown between these adjacent image pixels. That leads to image degradations. The image degradations are specially pronounced in bright areas of display when there are “bigger gaps” of gray scales between adjacent image pixels. It was observed in an image of a female model that there were artifacts shown on the forehead, the sides of the nose and the upper arm. The artifacts are generated by a technical limitation that the digitally controlled display does not provide a sufficient number of gray scales. At the bright spots of display, e.g., the forehead, the sides of the nose and the upper arm, the adjacent pixels are displayed with visible gaps of light intensities.
As the micromirrors are controlled to have a fully ON and fully OFF positions, the light intensity is determined by the length of time the micromirror is at the fully ON position. In order to increase the number of gray scales of display, the speed of the micromirror must be increased to the extent that the digitally controlled signals can be increased to a higher number of bits. However, when the speed of the micromirrors is increased, a stronger hinge is necessary for the micromirror to sustain a required number of operational cycles for a designated lifetime of operation. In order to drive the micromirrors supported on a further strengthened hinge, a higher voltage is required. The higher voltage may exceed twenty volts and may even be as high as thirty volts. The micromirrors manufactured by applying the CMOS technologies probably is not suitable for operation at such higher range of voltages and therefore the DMOS micromirror devices may be required. In order to achieve higher degree of gray scale control, a more complicated manufacturing process and larger device areas are necessary when DMOS micromirror is implemented. Conventional modes of micromirror control are therefore faced with a technical challenge that the gray scale accuracy must be sacrificed for the benefits of smaller and more cost effective micromirror display due to the operational voltage limitations.
There are many patents related to a light intensity control. These patents include the U.S. Pat. Nos. 5,589,852, 6,232,963, 6,592,227, 6,648,476, and 6,819,064. There are further patents and patent applications related to different shapes of light sources. These patents include the U.S. Pat. Nos. 5,442,414, 6,036,318 and Application 20030147052. The U.S. Pat. No. 6,746,123 discloses special polarized light sources for preventing a light loss. However, these patents or patent application do not provide an effective solution to overcome the limitations caused by insufficient gray scales in the digitally controlled image display systems.
There are several patents related to display systems that apply non-binary data for image control. These patents include the U.S. Pat. Nos. 5,315,540, 5,619,228, 5,969,710, 6,052,112, 6,970,148, and US Patent Application US 2005/0190429. Furthermore, there are many patents related to a spatial light modulation that includes the U.S. Pat. Nos. 2,025,143, 2,682,010, 2,681,423, 4,087,810, 4,292,732, 4,405,209, 4,454,541, 4,592,628, 4,615,595, 4,728,185, 4,767,192, 4,842,396, 4,907,862, 5,214,420, 5,287,096, 5,506,597, 5,489,952, 5,827,096, 6,064,366, 6,535,319, 6,719,427, 6,880,936, and 6,999,224. However, these inventions do not address or provide direct resolutions for a person of ordinary skills in the art to overcome the above-discussed limitations and difficulties.
Therefore, a need still exists in the art of image display systems applying digital control of a micromirror array as a spatial light modulator to provide new and improved systems such that the above-discussed difficulties can be resolved.